Omicron More Infectious in Younger People, People of African Ethnicity, Study States

Younger individuals and individuals of “African ethnicity” have higher rates of infection with Omicron than with Delta. That’s one of the findings of a new report by Imperial College London MRC Center for Global Infectious Disease which, while overall seems to paint a optimistic view of how the Omicron onslaught will play out, nonetheless also contains data that are cause for concern.

The authors observe that Omicron can significantly avoid immunity created by both prior infections and vaccines, although vaccine boosters appear to afford a much higher degree of protection. The report, which has not yet been peer reviewed, indicates that

Kevin Kavanagh, MD

rates of hospitalizations and asymptomatic infections are similar to Delta in those who have no preexisting immunity. As an Imperial College London press release accompanying the report notes, the risk of hospitalizations from Omicron is 15% to 20% less than for Delta in those who were never vaccinated or infected.

Omicron is felt to be 3 times as infectious as Delta, so a 15% reduction in hospitalizations can easily be overcome by increasing infection rates, a point underscored by Neil Ferguson, PhD, an internationally known epidemiologist and 1 of the authors of the report. “Given the high transmissibility of the Omicron virus, there remains the potential for health services to face increasing demand if Omicron cases continue to grow at the rate that has been seen in recent weeks,” said Ferguson, who is the director of Imperial College London’s MRC Centre for Global Infectious Disease Analysis and Jameel Institute.

That sentiment was echoed in the United States by Anthony Fauci, MD, the head of the National Institutes of Allergies and Infectious Diseases and the White House chief medical advisor, when he warned that large holiday gatherings pose a risk even for those who’ve been vaccinated and who’ve had a booster shot.

“Those are the kind of functions—in the context of COVID, and particularly in the context of Omicron—that you do not want to go to,” Fauci said at a White House briefing on Wednesday.

As reported in the Guardian, the BBC and various other news outlets, the Imperial College London’s “real-world” analysis for the risk of overnight COVID-19 hospitalization in the UK was reduced by 40% to 45%. Scotland found the risk of hospitalizations to be reduced by 65%.

However, these statistics are derived simply by determining the percent of hospitalizations in the total Omicron infections. The overall impact of Omicron in any country will be determined by the prevalence and strength of preexisting immunity in their population.

Another major caveat is that because of the immune escape potential of Omicron, the overall rosy results can be largely due to the inclusion of breakthrough and reinfections which are expected to cause less severe illnesses. And those who were thought to be protected by a 2-dose vaccine or a COVID-19 infection may be susceptible to severe disease because of the immune escape potential of Omicron and waning immunity. The first death of Omicron in the United States was in a previously COVID-19 infected individual.

The Imperial College Report estimated that vaccine effectiveness for prevention of symptomatic infections from Omicron for a 2-dose AstraZeneca vaccine to be between 0% to 20%. For a 2-dose Pfizer/BioNTech vaccine and 3-dose AstraZeneca vaccine the effectiveness was between 55% to 80%. The report suggests “very limited remaining protection against symptomatic infection afforded by 2 doses of AZ, low protection afforded by 2 doses of Pfizer, but moderate to high (55-80%) protection in people boosted with an mRNA vaccine.”

However, the effect of vaccine waning was not analyzed. The United Kingdom on average became fully vaccinated well after the United States. Many are looking at the total infections and see a much lower hospitalization rate, not realizing that there is a much larger number of cases and the expansion is largely due to reinfections and breakthrough infections which produce milder disease. The unvaccinated and those not previously infected are looking at a fate similar to Delta and those with waning immunity from vaccines or previous infections may endure a fate not that much better.

James Naismith, PhD, from the University of Oxford, noted in a press release, that while the study seems to be hopeful in terms of weathering Omicron, he did state that “Omicron can cause severe illness in the doubly vaccinated. Thus, if Omicron continues to double every few days, it could generate many more hospitalizations than Delta from the double vaccinated population. In my view the best news in the study is the observation that the booster is highly effective at reducing serious illness from Omicron. Put crudely we have more time to get more people boosted, we can’t waste a moment of it.

The Imperial College London report calculated that there was a 5.41 fold increase in reinfections as compared to Delta, suggesting that there are “relatively low remaining levels of immunity from prior infection.” However, even low levels of immunity may afford some protection against severe disease.

In another study, called Eave II, investigators looked at hospital data from November 23 to December 19, and found that the risk of hospitalization may be approximately two thirds lower with Omicron than Delta. Jim McMenamin, MBChB, the national COVID-19 incident director for Public Health Scotland, called the results “qualified” good news. He also warned that it was “important we don’t get ahead of ourselves.”

That study, which has also not been peer reviewed, concludes that the risk of hospitalization from Omicron may be 70% less when compared to Delta. But, the data were collected from a relatively small number of people, half of whom were between 20- and 39-years old. Only 6.6% were 60 years of age or older.

“The potentially serious impact of Omicron on a population cannot be underestimated,” McMenamin tells the Guardian. “And a smaller proportion of a much greater number of cases that might ultimately require treatment can still mean a substantial number of people who may experience severe COVID infections that could lead to potential hospitalization.”

In some ways the Scottish study is at odds with the Imperial College London report. The Scottish study looked at average hospitalizations for the Omicron wave as the Imperial College of London reported abundant data regarding a patient's risk of hospitalization based on their immunological status. The Imperial College London report stated that “we find no evidence (for both risk of hospitalization attendance and symptom status) of Omicron having different severity from Delta, though data on hospitalizations are still very limited.” The study also found that “18–29-year-olds, residents in the London region, and those of African ethnicity having significantly higher rates of infection with Omicron relative to Delta.”
 
Simply put, the Omicron wave is composed of abundant mild cases from individuals harboring SARS-CoV-2 immunity who should not have been infected in the first place, on top of cases in patients who are immunologically naive or who have waning immunity whose chance of hospitalizations is less but close to that of the Delta strain.

Both studies underscore the message of health policy experts that the public needs to increase its vigilance and public health measures to mitigate the spread of Omicron.

The bottom line is if we ignore the pandemic, it is possible that with the high infectivity of Omicron the sheer number of cases may overwhelm the world’s health care systems. No one knows for sure. But what is known is that everyone needs to obtain a booster if they are 6 months after the second dose of an mRNA vaccine, and to become vaccinated even if they had a previous infection.